Small, broadband, unidirectional antenna

ABSTRACT

A non-resonant antenna structure for the reception and/or radiation of electromagnetic waves in a unidirectional manner over a broad frequency band. The structure consists of two resonant elements of dual type fed simultaneously from the same transmission line. The resonant elements are adjusted so that the image impedance of the composite two-port structure is nearly constant and resistive over a very wide frequency band. One port is terminated in a resistor approximately equal in value to the image impedance. As a result, the impedance observed at the other port is also very nearly a constant resistance. In a given plane through the antenna one element has an omnidirectional radiation pattern; the other, a typical figure-of-eight dipole pattern. Proper adjustment of feeder impedance relative to the impedances of the individual elements leads to a unidirectional pattern with high front-to-back ratio over the operating band. Since achievement of the above conditions is possible at frequencies below the first resonant frequency of either element, the antenna dimensions may be small compared to the wavelength, particularly at the lower end of the operating band.

United States Patent [191 Mayes Jan. 9, 1973 [54] SMALL, BROADBAND,

UNIDIRECTIONAL ANTENNA [75] Inventor: Paul E. Mayes, Champaign, Ill.

[73] Assignee: JFD Electronics Corp., Brooklyn,

22 Filed: ot.13,1971

21 Appl. No.: 188,711

Primary Examiner-Eli Liberman' Attorney-Ostrolenk, FabenGerb & Soffen ABSTRACT A non-resonant antenna structure for the reception and/or radiation of, electromagnetic waves in a unidirectional manner over a broad frequency band. The structure consists of two resonant elements of dual type fed simultaneously from the same transmission line. The resonant elements are adjusted so that the image impedance of the composite two-port structure is nearly constant and resistive over a very wide frequency band. One port is terminated in a resistor approximately equal in value to the image impedance. As a result, the impedance observed at the other port is also very nearly a constant resistance. In a given plane through the antenna one element has an om nidirectional radiation pattern; the other, a typical figure-of-eight dipole pattern. Proper adjustment'of feeder impedance relative to the impedances of the individual elements leads to a unidirectional pattern with high front-to-back ratio over the operating band. Since achievement of the above conditions is possible at frequencies below the first resonant frequency of either element, the antenna dimensions may be small compared to the wavelength, particularly at the lower end of the operating band.

14 Claims, 28 Drawing Figures PATENTEDJAK 9197s 3.710.340

sum 1 OF 4 INVENTOR. 8404 IF. M14755 BACKGROUND OF THE INVENTION Broad band antennas are required in numerous electronic systems in use today and the prospects are that ever increasing bandwidths will be needed in the future. One example is in the reception of television broadcasts where a frequency span from 54 MHz up into the UHF band around 500 MHz is required in many communities today and the upper frequency limit will move upward toward 900 MHz as more UHF stations begin broadcasting. Present day high frequency (2-30 MHZ) communicationssystems require broadband antennas and land mobile service bandwidths have recently been increased.

In the years since 1959 log-periodic antennas have been used extensively to fill the requirement for broadband operation. Log-periodic antennas have unidirectional patterns of medium gain (4-10 dB above isotropic). The radiation patterns, gain, and input impedance of log-periodic antennas are very nearly constant over bandwidths in which the upper frequency limit may be 20 or more times greater than the lower end of the frequency range. However, log-periodic antennas are complex structures, usually containing many dipole-like elements and a long feeder to which all the dipoles are attached. Furthermore, since operation of the log-periodic antenna array depends upon the dipole elements being resonant somewhere in the operating band, they are often times bulky structures with attendant mechanical problems. These problems are parpreferably greater than the length of the slot. A single conductor transmission line is suspended or supported a'spaced distance (small compared to the wavelength) above and essentially parallel to the ground plane and essentially perpendicular to the long dimension of the slot. The total length of the transmission line may be only a fraction of a wavelength. At one end of the line a resistor is connected between the line and ground. The other end of the line becomes one input terminal of the antenna. The other input terminal is connected to the ground plane. Also connected to the transmission line, preferably at a location near the intersection of the center lines of the slot and preferably extending upward from the line is a conducting element which acts as a monopole above ground. The shape of the monopole may be one of several types, of which cylindrical and conical have been found satisfactory. In particular, a bi-triangular slot with a wide but shallow cavity, together with a wide-angle conical monopole have been found to provide very satisfactory performance characteristics, even to the point of minimizing the decreasing gain with decreasing frequency.

PRINCIPLE OBJECTS OF THE INVENTION It is therefore one primary object of the present invention to provide a novel antenna structure which has a predominantly resistive input impedance over a wide frequency band.

Another object of the present invention is to provide a novel antenna structure which has a unidirectional radiation pattern with a high value of front-to-back ratio over a wide frequency band.

Still another object of the present invention is to provide a novel antenna which operates satisfactorily even though the maximum dimension, particularly from the ticularly severe in the HF band where the constant gain characteristic of the log-periodic antenna may actually be unnecessary. I

BRIEF DESCRIPTION OF THE INVENTION The present invention is characterized by providing directional patterns and nearly constant impedance in a manner quite similar to the log-periodic antennas, but the maximum dimension thereof .is small compared to the longest wavelength of operation which is a substantial advantage, particularly in the HF and VHF bands. The reduction in size is obtained at the expense of constant gain, a feature which is not necessarily needed, particularly for receiving antennas and especiallyat frequencies below 100MHz.

The present invention is comprised of a conducting ground plane which may be on the surface of the earth or may be the surface of an aircraft or rocket. One radiating element is a slot which is cut out of the ground plane. The slot may be any of several different shapes although rectangular and bi-triangular (bowtie) configurations have been found satisfactory. A box with conducting walls is fastened over the slot on one side of the ground plane to prevent radiation on that side. It has been found that the depth of the box, commonly called a cavity, can be quite small compared to the longest wavelength of operation without seriously affecting the antenna performance. However, the width of the cavity must'exceed a minimum value (usually a few slot widths) and the length of the cavity is bottom of the cavity to the top of the monopole, is small compared to the longest wavelength of the operating range.

BRIEF DESCRIPTION or THE FIGURES These as well as other objects of the present invention will become apparent when. reading the accompanyingdescription and drawings in which:

FIG. 1 is a perspective view of one form of the antenna structure of the present invention showing a rectangular slot and a cylindrical'monopole.

FIG. 2 is a perspective view of an alternate form of the antenna structure showing a bi-triangular slot and a conical monopole.

FIG. 2a is a perspective view showing an alternative monopole which may be substituted for the monopole of FIG. 2. I

FIG. 3 isSmith Chart plot of the input impedance of an experimental model of the antenna. v I

FIGS. 4a-4y are measured radiation patterns corresponding to azimuthal variation of the vertical component of the electric field in the distant fieldof an experimental model of the antenna. Inthe figures like elements are designated by like numerals.

DETAILED DESCRIPTION OF THE INVENTION Making reference to the drawings; FIG. 1 shows an antenna A embodying the principles of the present invention comprising a slot 10 which is cut or otherwise formed in a conducting ground plane 1 1. A rectangular cavity 12 with conducting side walls 13a, 130 end walls 13b, 13d and bottom side 1362 is positioned to surround the perimeter of slot 10 on the underneath side of the ground plane, with the ground plane surrounding slot I the center-lines 18 and 19 of slot 10. The free end of transmission line 14 and the adjacent ground plane 11 form the input terminals of the antenna.

FIG. 2 shows an alternate form A for constructing the antenna of the subject invention wherein the rectangular slot 10 of FIG. 1 has been replaced by a bitriangular (i.e. bow-tie) slot 20 and the cylindrical monopole 17 of FIG. 1 has been replaced by a conical monopole 27. The monopole 27 may be either a solid conductive member or a hollow-conductive conical shell which may be either sealedor open-ended at its top end 27a.

As another alternative, the solid or hollow monopole 27 may be replaced with a wire mesh of conical shape. As still another alternative, the wire form of FIG. 2a may be employed. This embodiment 27 is comprised of a plurality of wires 28 whose lower ends are secured at a point 14a on conductive strip 14. The wires are aligned diagonally to generally conform to the desired conical shape. A single ring 29, which may be formed of wire, joins the upper ends of the wires 28. The mesh or wire formed embodiments significantly reduce the wind resistance of the monopole. Insofar as the embodiment 27 of FIG. 2a is concerned, it should be noted that the wires 28 actually employed may be greater or lesser in number that the number shown in FIG. 211.

FIG. 3 illustrates that the impedance observed at the input terminals of an experimental model of the antenna is indeed quite stable over a wide frequency band. The test results are illustrated for numerous frequencies between 100 MHz and 2,300 MHz for an antenna which had a 1.9 cm cavity depth D, 3.8 cm monopole height H and 17.8 cm slot length L (note FIG. 2, for example). The characteristic impedance of the feeder was 50-ohms and the line was terminated in a SO-ohm load (resistor 16). The VSWR with respect to 50-ohms does not exceed 2.0 in the band from below l MHz to above 2,000 MI-Iz, a ratio of band limits exceeding 20. Indeed the lower band limit is not established by impedance match since the input impedance will approach the terminal value (50 ohms) more and more closely as the radiating elements become smaller in terms of the wavelength.

FIGS. 4a-4y show that the lower frequency limit for this experimental model may be associated with patterns since the front-to-back ratio decreases below 150 MHz. Nevertheless, the remaining patterns indicate that the unidirectional character of the pattern is maintained up to 2,000MHz with the result that the pattern band. limit ratio of this particular model is only'slightly less than 20.

The results depicted in FIGS. 3 and 4a through 4y show how a structure of small size might be easily constructed for use as a receiving antenna for vertically polarized waves in the HF communications band (2-32 MHz). A commonly used antenna for like applications is a vertically-polarized log-periodic antenna wherein the largest element or supporting tower is about onequarter wavelength, which, at 2 MHz, is approximately 38 meters or 1 l 1.5 feet. Furthermore, the log-periodic array will be between one-half meters) and l.

wavelength (150 meters) long. By scaling the dimensions given above for use in an operating range from to 2,300 MHz, we see that an antenna of the present invention which is only 3 meters from bottom I of cavity (below ground) to top of monopole and 10 meters in width will provide comparable impedance and pattern performance for use in the range from 2-32 MHz. Constant gain is not required in this band because of the high levels of atmospheric noise.

Although this invention has been described with respect to its preferred embodiments, it should be understood that many variations and modifications will now be obvious to those skilled in the art, and it is preferred, therefore, that the scope of the invention be limited, not by the specific disclosure herein, but only by the appended claims. For example, the ground plane may be replaced by a wire mesh having a slot, or may be replaced by a plurality of spaced parallel wires arranged to define a ground plane and provided with a slot or otherwise bent to form the desired slot.

What is claimed is:

1. An antenna structure comprising:

a ground plane having an aperture;

an open ended enclosure with conducting walls attached to the ground plane in a manner to enclose the slot on one side of said ground plane, whereby the portion of said ground plane surrounding said slot engages the open end of the enclosure; single-conductor transmission line suspended a spaced distance above said slot and being substantially parallel to the ground plane and aligned transverse to one center line of said slot in the vicinity of the intersection of the center-line of the slot;

resistive termination electrically connected between the line and the ground plane being provided near one end of the line;

a monopole of conducting material electrically connected to said line near the point where the line passes over the slot;

terminal means at the other end of the line for connecting the antenna to transmission means for coupling to a transmitter or receiver.

2. The antenna of claim 1 wherein the slot has the shape of a rectangle.

3. The antenna of claim 1 wherein the slot has a bitriangular (bow-tie) shape.

4. The antenna of claim 1 wherein the monopole has a cylindrical shape.

5. The antenna of claim 1 whereinthe monopole has a conical shape.

6. The antenna of claim 4 wherein the monopole is constructed of a plurality of wires arranged to define the given shape.

7. The antenna of claim 6 wherein intersecting wires are provided to join said plurality of wires to cause said plurality of wires to retain the shape of the monopole.

8. The antenna of claim 4 where monopole is formed of a wire mesh.

9. The antenna of claim 1 wherein the ground plane is the surface of the earth.

10. The antenna of claim 1 wherein the ground plane a ground plane;

an aperture in said ground plane;

an elongated conductive strip positioned above said ground plane and overlying said slot;

a hollow open ended conductive enclosure positioned beneath said slot with said ground plane overlying the open end of said enclosure;

a monopole having its first end electrically coupled to said conductive strip;

a resistive element coupled between one end of said strip and said ground plane;

means provided on said ground plane. andthe opposite end of said strip and said ground plane for coupling said antenna to a transmitting or receiving instrument. I 

1. An antenna structure comprising: a ground plane having an aperture; an open ended enclosure with conducting walls attached to the ground plane in a manner to enclose the slot on one side of said ground plane, whereby the portion of said ground plane surrounding said slot engages the open end of the enclosure; a single-conductor transmission line suspended a spaced distance above said slot and being substantially parallel to the ground plane and aligned transverse to one center line of said slot in the vicinity of the intersection of the center-line of the slot; a resistive termination electrically connected between the line and the ground plane being provided near one end of the line; a monopole of conducting material electrically connected to said line near the point where the line passes over the slot; terminal means at the other end of the line for connecting the antenna to transmission means for coupling to a transmitter or receiver.
 2. The antenna of claim 1 wherein the slot has the shape of a rectangle.
 3. The antenna of claim 1 wherein the slot has a bitriangular (bow-tie) shape.
 4. The antenna of claim 1 wherein the monopole has a cylindrical shape.
 5. The antenna of claim 1 wherein the monopole has a conical shape.
 6. The antenna of claim 4 wherein the monopole is constructed of a plurality of wires arranged to define the given shape.
 7. The antenna of claim 6 wherein intersecting wires are provided to join said plurality of wires to cause said plurality of wires to retain the shape of the monopole.
 8. The antenna of claim 4 where monopole is formed of a wire mesh.
 9. The antenna of claim 1 wherein the ground plane is the surface of the earth.
 10. The antenna of claim 1 wherein the ground plane is a conducting mesh positioned near the surface of the earth.
 11. The antenna of claim 1 wherein the ground plane is the metallic skin of an aircraft, rocket, or vehicle.
 12. The antenna of claim 1 wherein the ground plane is constructed of a plurality of wires.
 13. The antenna of claim 1 wherein the transmission line is a conducting strip supported by a slab of dielectric material positioned between said ground plane and said conducting strip.
 14. An antenna comprising: a ground plane; an aperture in said ground plane; an elongated conductive strip positioned above said ground plane and overlying said slot; a hollow open ended conductive enclosure positioned beneath said slot with said ground plane overlying the open end of said enclosure; a monopole having its first end electrically coupled to said conductive strip; a resistive element coupled between one end of said strip and said ground plane; means provided on said ground plane and the opposite end of said strip and said ground plane for coupling said antenna to a transmitting or receivinG instrument. 